Starting from the “generics” project.
List Operations
record Pet(String species, String name, double weight) {}
List<Pet> xs = new Empty<Pet>();
xs = new Cell<Pet>(new Pet("cat", "Fluffy", 3.5));
xs = new Cell<Pet>(new Pet("dog", "Spot", 6.5));
xs = new Cell<Pet>(new Pet("cat", "Simba", 6.7));
xs = new Cell<Pet>(new Pet("dog", "Buddy", 4.5));
// heavier species total (cat or dog)
// totalSpeciesWeight
// Recursive version
// for (curr; !curr.empty(); curr = curr.rest()) version
A Quadratic Method
Design a function that takes two lists and returns a list of items that occur in both.
- Helper: recursive contains()
- Recursive: A O(n) function calls an O(n) function, so O(n^2)
- Or O(m*n)
- Direct iterative: Two loops, so O(n^2)
Introducing Map
Setup:
- A method that takes a
List<Pet>and ages each pet by one year. - A method that doubles each number in a list.
// Import
import java.util.function.Function;
// List
<U> List<U> map(Function<T, U> op);
// Empty
@Override
public <U> List<U> map(Function<T, U> op) {
return new Empty<U>();
}
// Map
@Override
public <U> List<U> map(Function<T, U> op) {
return new Cell<U>(op.apply(this.first()), this.rest.map(op));
}
Now rewrite our two example methods to use map.
Notes:
- The thing with “->” is called an anonymous function or a lambda function.
- It constructs an object with no name, of a type with no name, that implements the interface Function[T, U].
